Ion implantation is a standard technique for introducing conductivity-altering impurities into workpieces, such as semiconductor wafers. A desired impurity material is ionized in an ion source of an ion implanter, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material in the workpiece to form a region of desired conductivity.
An ion source is a critical component of an ion implanter. The ion source is required to generate a stable, well-defined ion beam for a variety of different ion species and extraction voltages. Electrons generated by the ion source will ionize a dopant gas to produce a plasma. This plasma may be formed into the ion beam used for implantation.
Large molecular compounds have been previously used in ion implanters. Carborane C2B10H12 is one example of a large molecular compound that may be used in ion implantation. Implantation of large molecular ions allows the equivalent of a high current, low energy atomic dopant ion implant with reduced energy contamination. Large molecules, such as C2B10H12, may have multiple dopant atoms per molecule. To obtain a specific dose, fewer large molecules are required than dopant atoms because each molecule may have multiple dopant atoms. Thus, for large molecules the dose or beam current may be reduced to attain a similar dose of dopant atoms or the dose may be increased at a particular beam current compared to that beam current of dopant atoms. Large molecule ions may obtain a similar depth as a dopant atom ion with higher energy due to the large molecule's size. This larger size may prevent channeling, or implantation substantially between the atoms of the crystal lattice of the workpiece. Thus, the beam energy may be increased for large molecule ions compared to dopant atom ions to obtain a similar implant depth. These higher beam energies reduce energy contamination in the beam and may limit the space-charge effect.
C2B10H12 and other large molecules present particular design challenges in an ion source. First, a large time between refills of a crucible, or recharge interval, in the ion source is desirable. C2B10H12 and other large molecules require a large reservoir of powder that is vaporized or sublimed to produce the vapor used in the ion source. A standard-sized crucible may not hold enough material to operate for long periods of time between recharges. Second, wall reactions may occur when material from the crucible condenses on a contacted surface. A material, such as C2B10H12 or another large molecule, may condense on non-heated surfaces. This condensation may lead to clogging or buildup on surfaces within the gas delivery system or the arc chamber. Third, pyrolysis may occur in an ion source. Pyrolysis is the decomposition of a compound or molecule by heating. Organic substances are one example of a compound or molecule that may decompose due to pyrolysis. The heat within a standard ion source may cause large molecules, such as C2B10H12, to break up or decompose. Lastly, it is difficult to switch species or have two crucibles with two different species operating in one ion source. In many designs, heat from operating one crucible to vaporize one species, such as arsenic, also may unintentionally vaporize the C2B10H12 or other large molecules in another crucible.
Accordingly, there is a need for a apparatus for ionization of large molecules in an ion implanter which overcomes the above-described inadequacies and shortcomings.